Particle detection

ABSTRACT

A particle detection system including; at least one light source adapted to illuminate a volume being monitored at least two wavelengths; a receiver) having a field of view and being adapted to receive light from at least one light source after said light has traversed the volume being monitored and being adapted to generate signals indicative of the intensity of light received at regions within the field of view of the receiver; a processor associated with the receiver adapted to process the signals generated by the receiver to correlate light received at least two wavelengths in corresponding regions within the field of view of the receiver and generate an output indicative of the relative level of light received at the two wavelengths.

FIELD OF THE INVENTION

The present invention relates to particle detection. It will beconvenient to hereinafter describe the invention in the context of smokedetection, however it should be appreciated that the present inventionis not limited to that use.

BACKGROUND OF THE INVENTION

Various methods of detecting particles in air are known. One method ofdetecting the presence of particulate matter in air involves projectinga beam across a monitored area and measuring the attenuation of thebeam. Such detectors are commonly known as ‘obscuration detectors’, orsimply ‘beam detectors’.

An exemplary, conventional beam detector is shown in FIG. 1. Thedetector 100 includes a light emitter and detector 102 and a reflector104 placed either side of a monitored area 106. Incident light 108 fromthe light emitter and detector 102 are projected toward the reflector104. The reflector 104 reflects the incident light 108 as reflectedlight 110. Reflected light 110 is reflected back toward the light sourceand detector 102. If particulate matter enters the monitored area 106,it will attenuate the incident light 108 and reflected light 110 andcause the amount of light received at the light source and detector 102to diminish. An alternative beam detector omits the reflector anddirectly illuminates the detector with the light source across themonitored area 106. Other geometries are also possible.

Whilst the mechanism of smoke detection used by beam detectors is sound,beam detectors commonly suffer from a number of problems.

Firstly, beam detectors may suffer a type I (false positive) error whereforeign objects or other particulate matter, such as dust, enters themonitored area and obscure the beam. Beam detectors are generally unableto distinguish between the obscuration caused by particles of intereste.g. smoke, and absorption which results from the presence of foreignbody of no interest e.g. a bug flying into the beam.

Secondly, beam detectors may require careful alignment at the time ofinstallation. Such alignment aims to ensure that in normal conditions,free from smoke, light enters the sensor so as to capture the majorityof the transmitted beam, and to in turn maximise sensitivity to anobscuration. This calibration may be slow and therefore costly toperform. Moreover, it may need to be repeated as the physicalenvironment that the detector occupies changes, for example because ofsmall movements in the structure to which a beam detector is attached.In some cases, if the intensity of incident light on the detectordiminishes quickly this misalignment may also cause a false alarm.

One way of compensating for the second problem is to introduce aphotodetector having a high sensitivity over a wide range of incidentangles. This reduces the effect that poor alignment between the beam andphotodetector would otherwise have. However, this solution comes at thecost of increased sensitivity to unwanted background light, which inturn complicates the detection process and increases the likelihood offailing to detect the presence of particles of interest.

Supplying power to the transmitters within a particle detection systemcan be costly. There are practical/commercial limits on the amount ofpower that can be supplied. The limited supply of power limits theoptical power output of the transmitter, which in turn limits the signalto noise ratio of the measured signal. If the signal to noise ratio ofthe system degrades too far, the system may experience frequent orcontinual false alarms.

In some systems, the signal to noise ratio can be enhanced by employinglong integration or averaging times at the receiver. However systemresponse times, which are usually between 10 and 60 seconds, must beincreased to higher levels if long integration times are used. This isundesirable.

SUMMARY OF THE INVENTION

In a first aspect the present invention provides a particle detectionsystem including; at least one light source adapted to illuminate avolume being monitored at least two wavelengths; a receiver having afield of view and being adapted to receive light from at least one lightsource after said light has traversed the volume being monitored andbeing adapted to generate signals indicative of the intensity of lightreceived at regions within the field of view of the receiver; aprocessor associated with the receiver adapted to process the signalsgenerated by the receiver to correlate light received at least twowavelengths in corresponding regions within the field of view of thereceiver and generate an output indicative of the relative obscurationof the light at the two wavelengths.

In one other aspect the present invention provides a particle detectionsystem including; at least one light source adapted to illuminate avolume being monitored at least two wavelengths; a receiver adapted toreceive light from at least one light source after traversing the volumebeing monitored and to generate an output that spatially and spectrallyresolves the received light; a processor to correlate light received atleast two wavelengths in corresponding spatial positions and generate anoutput indicative of the presence of particles in the volume beingmonitored.

Preferably the receiver includes a sensor having plurality of sensorelements. It can also include and image forming optics to form an imageincluding the at least one light source.

The light source can include a one or more of light emitters adapted toemit light at a respective wavelength. A light source can emit at asingle wavelength only, or a plurality of wavelengths.

The light source can illuminate the volume being monitored at each ofthe at least two wavelengths at different times. Alternatively the lightsource can include a light emitter adapted to emit light over a broadwavelength band including the at least two wavelengths simultaneously.

Preferably the particle detection system includes a plurality of lightsources.

The processor can be adapted to determine a relative intensity of lightreceived at least two wavelengths in corresponding spatial positions andgenerate an output indicative of the presence of particles in the volumebeing monitored.

The commissioning of such a system can include approximately aligningthe light source(s) and receiver such that the at least one light sourceilluminates the receiver, and selecting in the image sensor whichspatial position corresponds to the light source and will be used formeasuring received light intensity measurements corresponding to thelight source. As the geometry of the system will drift over time theprocessor preferably tracks which spatial position corresponds to thelight source over time.

The beam of light can be formed using a light source located remotelyfrom the light sensor and positioned to emit light at one or morewavelengths across the monitored region. The beam of light can be formedwith one or more reflective targets adapted to reflect a beam of lightfrom a light emitter across the monitored region. In this arrangementthe light emitter can be mounted nearby the light sensor and thereflective target located remotely.

A system can include a plurality of beams received on a common lightreceiver.

In another aspect the present invention provides a beam detector fordetecting particles of interest within a monitored volume, said detectorincluding:

at least one light source for projecting light across a monitoredregion, said light including a plurality of wavelengths including atleast a first wavelength which is relatively unaffected by particles ofinterest, and at least a second wavelength that is affected by at leastsaid particles;a receiver for receiving at least a portion of said projected light andoutput a signal indicative of an intensity of light received from saidlight source at least the first and second wavelengths; anda controller adapted to process the output of the receiver at least oneof the first and second wavelengths and provide an output indicative ofwhether particles of interest are detected in said monitored region.

Of course, it will be understood that “first wavelength” and “secondwavelength”, may indicate a wavelength component that is emitted by of abroad spectrum radiation emitter but can also be used to denote arelatively narrow wavelength band by reference to one wavelength withinit (usually the central wavelength) such as would be emitted by anemitter with a narrow passband like a laser diode or LED etc, e.g. afirst wavelength band may be in the infrared and be centred at 850 nmand have a bandwidth of 50 nm.

As will be appreciated, whilst the illustrative examples relate to theuse of visible or near visible electromagnetic radiation the tem lightcan be seen as broadly encompassing the electromagnetic spectrum.However in the visible and near visible portions of the EM spectrum thechallenges in practically and cheaply generating, controlling, focussingand receiving are minimised.

In this way the received light level at the first and second wavelengthscan be used to distinguish between the presence of particles of interestand changes in received light levels caused by other factors.

The light source can selectively (e.g. temporally, spatially orspectrally) project light at the at least two wavelengths.Alternatively, the light source can project a light with a widebandwidth, e.g. white light, that includes light at least the first andsecond wavelengths. In a system with a wide bandwidth light source thereceiver may cooperate with coloured filters to receive and discriminatebetween the at least two wavelengths.

Preferably the relative intensity of the received light level at the atleast two wavelengths is determined e.g. the ratio or difference betweenthem. In the event that the relative intensity of light remainssubstantially the same, a change in the received light level can beattributed to a factor other than the presence of particles of interestin the monitored region. If a fault condition is met a fault can besignalled.

In the event that a change in received light level at one or bothwavelengths causes the relative intensity of light to change in apredefined manner the change in received light level can be attributedto the presence of particles of interest in the monitored region. If analarm condition is met, a particle detection alarm can be raised.

Preferably the first wavelength is in the infrared portion of theelectromagnetic spectrum. The second wavelength is preferably in theultraviolet portion of the electromagnetic spectrum.

The illumination at the first and second wavelengths is preferablyperformed alternately. The alternating illumination can be interspersedwith periods of no illumination.

In another embodiment a second alarm condition that is determined on thebasis of the received light level at one or both wavelengths is alsodefined such that, in the event that a change in received light level atone or both wavelengths does not cause the relative intensity of lightto change in manner that causes the first alarm condition to be met, thesecond alarm condition can be met.

Preferably the second alarm condition is based on the value of thereceived light level at one or both wavelengths. Most particularly thesecond alarm condition compares value of the received light level at oneor both wavelengths to a threshold. The second alarm condition can bedetermined on the basis of the rate of change of the received lightlevel at one or both wavelengths.

This aspect of the invention also provides a method of detectingparticles in a region being monitored, including:

emitting light including a first and second wavelength into the regionbeing monitored; the first wavelength being a wavelength whosetransmission across the monitored region is relatively unaffected byparticles of interest and the second being a wavelength whosetransmission across the monitored region is affected by particles ofinterest;receiving light at least a first and second wavelengths after traversingthe region being monitored and generating a signal indicative of theintensity of the received light at least the first and secondwavelength;processing the signal indicative of the intensity of the received lightat least the first and second wavelength to provide an output indicativeof whether particles of interest are detected in said monitored region.

The step of processing the signal indicative of the intensity of thereceived light at least the first and second wavelength can be based ona change in the relative intensity of the light received at the firstand second wavelengths.

In the event that the relative intensity of light at least the first andsecond wavelength changes in a predetermined manner an output indicatingthe presence of particles of interest in said monitored region can bemade. Preferably the changes in relative intensity at the twowavelengths are compared to a threshold and if the change in relativeobscuration exceeds the threshold an alarm condition is indicated. Thethreshold can be user selected but is preferably reflects a differencein obscuration at the two wavelengths of between 10% and 50%.

In the event that the relative intensity of the light received at thefirst and second wavelengths remains substantially stable but anabsolute intensity of the light received at one or more of thewavelengths meets one or more predetermined criteria an outputindicating the presence of particles of interest in said monitoredregion can be made.

Another aspect of the present invention provides a beam detectorincluding means for projecting light across a monitored region; meansfor receiving said light; and processing means; said means for receivingbeing adapted to discriminate between at least two wavelengths withinsaid light; the processing means being configured to provide a signalindicative of particles in the monitored region in response to arelative intensity of received light at the at least two wavelengths;and the processing means providing a signal indicative of particles inthe monitored region in response to a received light level at least onewavelength, preferably one of the at least two wavelengths.

This aspect of the present invention also provides a method of detectingparticles in a monitored region; including:

-   -   measuring a received light level at least two wavelengths to        determine particle concentration,    -   determining whether at least one first particle detection        criteria is satisfied based on a relative intensity of received        light at the at least two wavelengths, and    -   determining whether at least one second particle detection        criteria is satisfied based on a received light level at least        one wavelength.

In a further aspect the present invention provides a receiver for aparticle detection system, said receiver having a field of view andbeing adapted to receive light at least two wavelengths, from at leastone light source, which has traversed the volume being monitored, thereceiver being configured to generate signals indicative of theintensity of the received light at region within the field of viewcorresponding to each light source at least one or a plurality of thewavelengths. The receiver preferably has an associated processorconfigured to process the signal indicative of the intensity of thereceived light at two or more wavelengths to determine the relativeobscuration of the light received from the at least one light source atthe two wavelengths. The receiver can include a sensor having pluralityof sensor elements each adapted to receive light from a respectiveregion within the field of view of the receiver, .e.g. a video camera orsimilar imaging device. The receiver can receive light from a lightsource at least two wavelengths in the same region. Alternatively thereceiver could receive light from two light sources at differentwavelengths in different regions, and determines the relativeobscuration of the light received from the two light sources at thedifferent wavelengths.

In a further aspect the present invention provides a receiver for aparticle detection system, the receiver including a light sensor, thelight sensor having field of view and being capable of distinguishinglight received from a plurality of regions within the field of view attwo or more wavelengths; and a processor adapted to receive, from thelight sensor, data representing received light and to identify at leastone region of the plurality of regions, in which light from a respectiveone of one or more light sources is received; said processor providing asignal indicative of particles in the monitored region on the basis ofthe relative level of received light at least two wavelengths in theidentified region of the plurality of regions.

Preferably the processor is adapted to update said identification of theat least one region over time. Preferably the light sensor includes aplurality of light sensor elements, e.g. pixels, each of whichcorrespond to a respective region of the field of view. The processorcan be adapted to identify a subset including one or more light sensorelements at which light from the light source is received. The processormay process the received data at successive time periods and trackchanges in the subset of the sensor elements corresponding to one ormore light sources over time.

Advantageously, this arrangement can possess the advantage of a widefield sensor in terms of ease of alignment, and the advantage of narrowviewing angle sensors in terms of receiver noise.

The commissioning of such a system can include approximately aligningthe beam and the light sensor, such that the beam falls on the sensor,and performing an image sensor element selection process to determinewhich image sensor elements will be used for taking received lightintensity measurements. As the geometry of the system will drift overtime the processor can track which image sensor element(s) are receivingthe beam over time.

The beam of light can be formed with one or more reflective targetsadapted to reflect a beam of light from a light emitter across themonitored region. In this arrangement the light emitter can be mountednearby the light sensor and the reflective target located remotely.Indeed the receiver may include one or more transmitters for projectinglight toward one or more reflective targets, said targets forming saidlight sources.

A system can include a plurality of beams received on a common lightsensor.

Each light source may include one or more bandpass filters toselectively emit light in chosen wavelength bands.

This aspect of the invention also provides a particle detection systemincluding such a receiver and at least one light source for cooperatingwith the receiver to define at least one beam detector. Preferably thesystem includes at least one other beam detector and control means(which may be wholly or partly formed by the processor) configured to:

-   -   detect particles using the first beam detector;    -   determine if particles are detected by at least one other beam        detector; and    -   determine the location of the detected particles on the basis of        said determination and the relative positions of first the first        beam detector and the at least one other beam detector.

The at least two beam detectors could simply be two light sourcescooperating with a common receiver.

Preferably, in the event that particles are also detected by the atleast one other beam detector the location of the particles isdetermined to be a region monitored by both beam detectors.

In the event that particles are not detected by the other beam detectorsthe location of the particles is determined to be a region monitored bythe first beam detector but not the other beam detectors.

Preferably the beam detectors are arranged such that at a plurality oflocations in the region being monitored by the system are monitored byat least two beam detectors.

The system can include a plurality of beam detectors arranged to monitorintersecting regions.

Most preferably the particle detection system includes a first receiveradapted to monitor obscuration of a plurality of beams to thereby definea corresponding plurality of beam detectors.

In one embodiment the system includes two receivers each monitoring aplurality of beams to thereby define two groups of beam detectors, andwherein at least one of beam detectors of each group monitor a commonlocation. Preferably each beam of each group monitors at least onelocation monitored by a beam detector of the other group.

The particle detection system can include beam detectors having beampaths with differing lengths. Preferably at least two beam detectors arearranged next to each other such that their lengths overlap to enable alocation of particle detection, along the length of the beam of thefirst detector, to be determined.

Preferably the particle detection system includes a light receiveradapted to receive a plurality of light beams. The detection system caninclude a plurality of light receivers adapted to receive a respectiveplurality of light beams.

Preferably the light receivers and beams are arranged such that one ormore beams pass nearby at least one other beam at known locations toenable localisation of a particle detection event to one of suchlocations in the event that particles are detected on at least one pairof beams.

This aspect of the invention also provides a method of commissioning aparticle detector including a plurality of light sources and a lightreceiver, the light receiver including a light sensor, the light sensorhaving field of view and being capable of distinguishing light receivedfrom a plurality of regions within the field of view; the methodincluding: arranging the light receiver such that the plurality of lightsource are in the field of view of the light receiver; and identifying,on the basis of the output of the light receiver, at least one region ofthe plurality of regions, in which light from a respective at least twoof one or more light sources is received to define a plurality ofnotional beam detectors and independently determining whether particlesare detected using each of the notional beam detectors.

The method can include allocating an address on a fire alarm systemcorresponding to each the notional beam detector defined by a lightsource in the field of view of the receiver and the receiver.

The method can include positioning one or more reflectors, thereflectors forming light sources and being adapted to reflect light froma light emitter.

In a further aspect the present invention provides a particle detectionsystem incorporating a plurality of beam detectors each having arespective beam transmitted along a corresponding beam path, and whereinthe beam paths of at least two of the beam detectors have a region ofsubstantial coincidence, such that in the event that particles aredetected in two beams the position of the detected particles can bedetermined to be within the region of substantial coincidence.

Preferably, in the event that particles are detected in one of the twobeams, but not the other, the position of the detected particles can bedetermined to be at a position within the beam on which detectionoccurred, but outside the region of substantial coincidence.

In one exemplary system the region of substantial coincidence of twobeams is a crossing point of the beams. Alternatively the beams canproject parallel to each other and overlap for part of the length of oneof at least one of the beams, and the region of substantial coincidencecan be the region where the beams overlap.

Preferably the plurality of beam detectors shares either a light sourceor a light receiver.

In any of the above embodiments more than one spatially separated lightsources, reflectors or beams can be used.

-   -   In another aspect there is provided a particle detector        configured to detect particles of interest in a region being        monitored, the detector including:        -   remote illumination means adapted to emit light at one or            more first wavelengths to illuminate at least part of the            region being monitored;        -   second illumination means adapted to emit light at one or            more second wavelengths to illuminate at least part of the            region being monitored;        -   a receiver configured to receive a portion of the emitted            light at the first and second wavelengths after traversing            the region being monitored, said receiver being            substantially co-located with the second illumination means;            and        -   at least one reflector located remotely from the receiver            and arranged to reflect the light emitted from the second            illumination means to the receiver.

Preferably the reflector and remote illumination means are substantiallyco-located. Most preferably they are housed in a common device.

Preferably the receiver and the second illumination means are housed ina common device.

The remote illumination means is preferably battery powered. Theillumination means preferably includes one or more light sources. Mostpreferably the light sources are LEDs.

The system can include a plurality of either remote illumination meansand/or reflectors.

In another aspect the present invention provides a device incorporatingremote illumination means and reflector for use in such a system.

A light source for a particle detection system, said light sourceincluding at least one emitter light emitter adapted to project a beamof light; housing supporting the light emitter and mounting meansenabling attachment of the housing to a support structure, the mountingmeans being coupled to the housing such that the orientation of thehousing can be changed with respect to the support structure on whichthe light source is supported.

The light source or receiver can additionally include an indicator forindicating the relative orientation of the direction of projection ofthe light beam and either or both of: the support structure on which thelight source is supported; or an axis of the mounting means.

The indicator can include a dial having one portion indicating angularorientation with respect to an axis of the mounting means and anotherportion indicating angular orientation with respect to the direction ofprojection of the light beam.

The light source or receiver can be configured to cooperate withremovable sighting means to be used for alignment of the light sourcewith respect to the receiver.

A method of indicating alignment of a light source and a receiver in abeam detector, said light source being configured to emit two partiallyoverlapping beams of light to be received by the receiver, the methodincluding:

-   -   modulating a first beam of the overlapping beams of light with a        first modulation scheme;    -   modulating a second beam of the overlapping beams of light with        a second modulation scheme that is distinguishable from the        first modulation scheme;    -   receiving light from the light source;    -   determining the relative alignment of the light source and the        receiver in the basis of a modulation scheme detected in the        received light.

Preferably the method includes indicating correct alignment of the lightsource and receiver if a component of the received light is modulatedaccording to the each of the first and second modulation schemes.

Preferably the method includes indicating misalignment if the receivedlight is modulated according to only one of the first and secondmodulation schemes.

A method of detecting a condition of a light source of a particledetection system which emits a beam of light received by a receiver,said method including:

-   -   modulating the illumination of the light source according to a        predetermined modulation scheme:    -   varying the modulation scheme in the event that a predetermined        condition exists in the light source;    -   detecting the variation in the modulation scheme in the light        received by the receiver.

Preferably the condition indicated is a low battery condition in thelight source.

The method can include intermittently varying the modulation schemebetween the predetermined modulation scheme and the varied modulationscheme.

A method for detecting particles in a region, comprising;

-   -   providing a receiver having a field of view insufficient to view        the entire region;    -   forming a plurality of beams projecting across the region        towards the receiver;    -   changing the orientation of the field of view of a receiver to        monitor a plurality of the beams; and    -   determining whether particles are present in the region on the        basis of each the level of light received from each received        beam.

The step of forming a plurality of beams projecting across the regiontowards the receiver can include projecting beams across the region tocoincide with field of view of a receiver as it changes. The beams canbe formed directly by a light source or by reflecting a light sourcefrom a reflector.

In a preferred form the method includes scanning the field of viewthrough a predetermined angle to sequentially receive light from aplurality of beams. The method can include scanning a light sourceacross the region in time with the field of view of the receiver, andreceiving light beams reflected from a plurality of reflectors.

In a further aspect the present invention provides a method ofmonitoring for particles in a region using a particle detector of thetype described above, said method including:

-   -   illuminating at least part of the region being monitored using        the remote illumination means;    -   receiving at least a portion of the emitted by the remote        illumination means after traversing the region being monitored,        and in the event that the received light level meets at least        one predetermined criteria;    -   illuminating at least part of the region being monitored using a        second illumination means;    -   receiving at least a portion of the emitted light at a second        wavelength after traversing the region being monitored, and    -   determining whether particles are present in the region being        monitored on the basis of a received signal at one or both of        the wavelengths.

In an aspect the present invention provides a light source for use in aparticle detector said light source including:

-   -   a plurality of light emitting elements arranged to project a        light beams in respective directions;    -   means for selectively illuminating one or more of the light        emitting elements such that the light source can be configured        to project at least one selected direction.

Preferably the light emitting elements are LEDs.

Preferably the light emitting elements have relatively narrow fields ofillumination and are arranged such that light source may have arelatively wide field of illumination. Preferably the field ofillumination of each light emitting element at least partially overlapsthat of another light emitting element.

A method in a particle detector including a light source of the previousaspect of the present invention to generate a beam of light, the methodincluding:

-   -   determining a desired direction of projection of the beam of        light; and    -   selectively illuminating one or more of the light emitting        elements which project a beam of light in the desired direction.

The method can include, illuminating a one or more of the light emittingelements and monitoring for reception of a beam of light at a receiver;and in the event that a beam of light is not received, selecting anotherlight emitting element for illumination. This step can be repeated untila beam of light is detected.

In the above embodiments each light source can be adapted to generate anillumination at a plurality of wavelengths, preferably two wavelengths,to enable an embodiment of any one of the particle detection methodsdescribed herein to be performed.

In the above embodiments a light source can be adapted generate light attwo wavelengths according to a modulation scheme. The scheme can includea pulse train which includes at least one pulse of light at a firstwavelength and pulse of light at a second wavelength. A plurality ofpulses at one or both of the wavelengths can be included in a pulsetrain. In the event that a plurality of light sources is used themodulation pattern of the light sources may be the same or different.Moreover the modulation pattern of the light sources are preferably notsynchronised with each other.

BRIEF DESCRIPTION OF THE DRAWINGS

Illustrative embodiments of the present invention will now be describedby way of non-limiting example only with reference to the followingfigures, in which:

FIG. 1 is a prior art beam detector:

FIG. 2 illustrates a first embodiment of the present invention:

FIG. 3 a and FIGS. 3 b illustrate schematically an image received at thelight sensor of the light receiver 204 of the system in FIG. 2;

FIG. 4 illustrates a second embodiment of the present invention usingtwo wavelengths of light;

FIGS. 5 a and 5 b illustrate schematically the operation of the detectorof FIG. 4 in two circumstances:

FIG. 6 illustrates a further embodiment of the present invention whichincludes two targets in the field of view of the receiver;

FIG. 7 illustrates a further embodiment of the present invention whichdoes not include a target;

FIG. 8 illustrates a further embodiment of the present invention havingsix light beams spanning a monitored area;

FIG. 9 illustrates a particle detection system illustrating anaddressing scheme according to a further aspect of the presentinvention;

FIG. 10 illustrates a particle detection system with a second addressingscheme according to an embodiment of the present invention;

FIG. 10A illustrates a retroreflective target;

FIG. 11A illustrates a plot of received light intensity for a particledetector operating at two wavelengths when detecting products ofcombustion demonstrating an unusually high proportion of large particlescompared to small particles;

FIG. 11B is a plot of a comparison of detector output at first andsecond wavelengths corresponding to FIG. 11A;

FIG. 12A illustrates a detector output at three wavelengths for productsof combustion demonstrating an unusually high proportion of largeparticles compared to small particles;

FIG. 12B is a plot of the two wavelength comparison between the firstwavelength and the third wavelength from FIG. 12A;

FIG. 13 illustrates how an alarm threshold may be implemented in anembodiment of the present invention;

FIG. 14 illustrates a beacon used in an embodiment of the presentinvention;

FIG. 15 shows a schematic view of the beacon of FIG. 14;

FIG. 16 shows a schematic side view of the variant of the beacon of FIG.14;

FIG. 17 shows two encoding schemes able to be used by a beacon in anembodiment of the present invention;

FIG. 18 illustrates a particle detection system according to a furtherembodiment of the present invention, which uses plurality stationarybeacons and a scanning detector to cover a 90° field of view;

FIG. 19 is a schematic representation of the mechanical system used in ascanning receiver and light source arrangement of an embodiment of thepresent invention;

FIG. 20 illustrates a particle detection system according to a furtherembodiment of the present invention, which uses a scanning camera andlight source arrangement to cover a 360° field of view;

FIG. 21 illustrates a beacon according to an embodiment of the presentinvention with an alignment mechanism;

FIG. 22 shows the top view of the beacon of FIG. 21;

FIG. 23 shows another means for aligning a beacon in an embodiment ofthe present invention;

FIG. 24 shows a bottom view of the alignment means of FIG. 23;

FIG. 25 shows a beacon according to a further embodiment of the presentinvention;

FIG. 25A shows a beacon according to a further embodiment of the presentinvention;

FIG. 26 illustrates a further beacon usable in another embodiment of thepresent inventions.

FIG. 27 illustrates a schematic block diagram of a receiver component ofbeam detector according to an embodiment of the present invention; and

FIG. 28 illustrates an exemplary pulse train used in an embodiment ofthe present invention.

DETAILED DESCRIPTION OF THE EMBODIMENTS

FIG. 2 shows an embodiment of the present invention. The detector 200includes a light emitter 202, a receiver 204, and a target 206, actingin co-operation to detect particles in a monitored area 208. Target 206reflects incident light 210 and thereby forms a light source and returnsreflected light 212 to receiver 204. Preferably the target is a cornercube or other reflector adapted to reflect light back along its incidentpath, or other determined path.

The term light source as used is intended to be interpreted to include adevice that actively produces an illumination from one or more(generally termed a light emitter or transmitter herein) as well as areflector of an illumination generated by another device (generallytermed a target or reflector herein).

In the preferred embodiment the receiver 204 is preferably a videocamera or other receiver having an array of light sensors. A personskilled in the art would appreciate that receiver 204 may be constructedusing a range of image sensor types, including one or more CCD(charge-coupled device) image sensors, or CMOS (complementarymetal-oxide-semiconductor) image sensors, or indeed any device capableof recording and reporting light intensity at a plurality of pointsacross its field of view, without departing from the spirit of theinvention.

Receiver 204 receives all of the light in its field of view 220, andincludes imaging optics to form an image of a field of its view 220,including the target 206 on its image sensor. This light includesreflected light 212. Receiver 204 records the intensity of all light inits field of view, in the form of data representing the image intensityat a series of locations throughout the field of view. A portion of thisdata will correspond, at least partially, to reflected light 212.Receiver 204 communicates the image data to a microcontroller. Themicrocontroller analyses the image data, and determines which portion ofthe data provides the best estimate of reflected light 212. Because thereceiver 204 has a wide field of view and has the ability to measurelight at a wide range of points within this field of view the lightemitter 202 need not be carefully aligned with target 206, or withreceiver 204, since the effect of a misalignment will simply be that adifferent portion of data, corresponding to different pixels within theview, will be used a measure of reflected light 212. Accordingly,provided that the field of view of the receiver includes target 206, oneor more regions of interest within the image will include a measuredvalue for the reflected light 212. It is noted that additionalbackground or stray light from areas other than the region of interestcan be ignored by the microcontroller.

The microcontroller may base its decision, as to which pixels of theimage sensor correspond to the reflected light 212 for example, on therelative intensity of a particular part of the image compared with otherareas of the image. It may similarly use information gained about theenvironment, or historically recorded data. At the conclusion of thisdecision process, the microcontroller will have selected a portion ofdata, perhaps corresponding to a pixel or group of pixels read from theimage sensor, that can most reliably be used to measure the intensity ofreflected light 212.

The microcontroller now monitors the regions of the image that it haspreviously selected as corresponding to the reflected light 212. Ifsmoke or other particulate matter enters monitored area 208, smoke orparticulate matter will obscure or scatter incident light 210 orreflected light 212. This obscuration or scattering will be detected asa drop in the intensity for received reflected light 212 measured in theimage region determined by the microcontroller.

Pixels falling outside the region selected by the microcontroller, toinclude the reflected light 212, can be ignored as light received bythese pixels does not correspond to the reflected light 212.

Over time, as the building moves or other factors alter the geometry ofthe system, the target 206 will still be in the field of view of thereceiver 204 however, the image of the target 206 will appear at adifferent point on the image detector of the receiver 204. In order toaddress this motion of the image of the detector, the microcontroller isadapted to track the image of the target 206 across its light sensorover time to enable a smoke detection to be performed on the correctimage regions over time.

FIG. 3 a and FIG. 3 b illustrate schematically an image received at thelight sensor of the light receiver 204 of the system in FIG. 2 atdifferent times. In this embodiment the output of the sensor enables thereceived light intensity at a plurality of locations to be determined.In one form the sensor is a CMOS imaging chip or similar, and includes aplurality of pixels 302, each pixel corresponding to a position in thefield of view 300 of the light receiver. In use, the microcontrollerreads out the light intensity of the plurality of the pixels e.g. 302.In any given image frame the light level received varies from pixel topixel within the array of pixels 300.

By analysing the image, the microcontroller can determine that certainpixels (or a single pixel) correspond to an image of the target 206,which lies with in the field of view of the receiver 204. This group ofpixels, labelled 304, has a substantially higher level of received lightthan the other pixels and corresponds to the received beam transmittedby the light source.

Over time, as the building moves or other factors alter the geometry ofthe system, the target 206 will still be in the field of view of thereceiver 204 however, the image of the target 206 will appear at adifferent point on the image detector of the receiver 204. In order toaddress this motion of the image of the detector, the systemmicrocontroller can be adapted to track the image of the target 36across its light sensor over time to enable particle detection to beperformed on the correct image regions over time. FIG. 3 b issubstantially identical to FIG. 3 a with the exception that the “spot”caused by the target in the field of view 300 has moved in a directionindicated by arrow 310.

In one embodiment, tracking of the “spot” can be performed by themicroprocessor initially storing in memory a first set of pixelco-ordinates corresponding to the “spot” in the field of view.Periodically, the microcontroller examines the measured value of thepixels within a predetermined distance from the “spot”, including thepixels corresponding to the spot. The microcontroller then calculates asecond list of pixel co-ordinates by selecting the n-brightest pixelsfrom the said surrounding area. The second list is then compared withthe first list, and if the lists differ by more than m-pixel co-ordinatepairs, an error is indicated. If the lists differ by m-less or pixelcoordinate pairs, the second list of pixel co-ordinates is stored inplace of the first list of pixels.

In an alternative scheme the controller of the system can analyse thereceived image, and determine which portion of the image containsinformation most strongly related to a received beam. At the conclusionof this decision process, the controller will have selected two portionsof signals that are produced by respective individual sensors or groupsof sensors, so the selected signal can most reliably be used to measurethe intensity of beams. One way of selecting the sensors whose data canbe most reliably used is to view the image generated by the receiver atthe time of commissioning the smoke detector and selecting theappropriate sensors.

A further mechanism of ensuring that the calculated received beamintensity is as close to the actual intensity of the received beam aspossible can involve the microcontroller deciding whether to use thevalue corresponding to a certain sensor element, according to thatelement's contribution to the overall image strength. For example, fromthe sensor element outputs, the controller can determine a‘centre-of-signal’ position of the beam. The centre-of-signal positionis analogous to the centre of mass position, except that instead ofmass, it is the signal value contributed by each pixel (i.e. sensorelement) that is used in the calculation. For example, the followingequation may be used:

Centre-of-signal position vector={sum of (position vector of eachpixel)*(value of each pixel)}/{sum of values from all the pixels}.

After the centre-of-signal position is determined, the controller mayweight the signal contributed to the received beam intensity value byeach sensor element (i.e. corresponding to the electrical signalgenerated by each sensor) according to the distance between that sensorelement. In this way, the controller determines the sensor elementswhose signals best represent the target image and that are least likelyto be dropped from subsequent measurements due to drift in the beamimage's position on the sensor.

In use the microcontroller will compare the intensity of light receivedwithin this group of pixels to the light received in an earlier image todetermine whether there had been an increase in obscuration of the beamcaused by particles in the monitored region 208.

The microcontroller can then use conventional smoke detection methods todetermine when smoke is detected and if an alarm should be raised. Forexample smoke can be detected by monitoring the level of received light,and when a chosen characteristic of the received light meets one or morepredetermined criteria it is determined that smoke is present in thevolume being monitored. For example, when the received light level fallsbelow a predetermined level it can be determined that smoke is present.Alternatively, when the rate of change of the received light levelexceeds a predetermined level it may be determined that smoke isdetected. As will be appreciated the smoke detection criteria can alsoinclude a temporal condition, e.g. that the received light level mustdrop below a threshold for more than predetermined period of time beforean alarm is raised.

To improve the system's sensitivity, a cancellation algorithm can beused to minimise the effect of background light on the measured receivedlight intensity. One such algorithm operates by alternatively capturingimages of the field of view of the receiver with the light source turnedon and off. The measured light intensity of the “off” frames (i.e.images captured without illumination) can be subtracted from the “on”frames (i.e. images captured with illumination). Received light that isnot attributable to the illumination by the light source backgroundlight, can thereby be substantially eliminated.

A person skilled in the art would appreciate that collection of ‘offframes’ can be achieved in a variety of ways, including by selectivelysuppressing a light source having a particular wavelength, for exampleby modulating a control input to the light sources, or alternatively byintroducing a filter in front of the source that temporally blocks lighthaving particular wavelengths. Such a person would also appreciate thatelimination of background light could be achieved by means other thansimple subtraction, for example by use of suitable filter, or by someother computational approach.

In a preferred embodiment of the present invention the target isilluminated at two (or more) wavelengths. FIG. 4 depicts an embodimentof the present invention having multiple light emitters which emit lightat two wavelengths λ₁ and λ₂. This example includes an infrared (IR)light emitter and ultraviolet (UV) light emitter 402 which emit lightalong two substantially collinear paths. It also includes a receiver404, and a target 406, acting in co-operation to detect smoke inmonitored area 408. Target 406 reflects incident UV light 410 asreflected UV light 414 and also reflects incident IR light 412 asreflected IR light 416. The two wavelengths are chosen such that theydisplay different behaviour in the presence of particles to be detected,e.g. smoke particles. In this way the relative change in the receivedlight at the two (or more) wavelengths can be used to give an indicationof what has caused attenuation of the beam.

Receiver 404 receives both reflected infrared light 416 and reflectedultraviolet light 414, along with other light in its field of view.Receiver 404 records the intensity of all light in its field of view ata series of locations throughout the field of view as described above. Aportion of this data will correspond, at least partially, to theintensity of reflected infrared light 414. A portion of this data willcorrespond, at least partially, to the intensity of reflectedultraviolet light 414. Receiver 404 includes microcontroller 424 forprocessing image data.

In this system, to apply the background cancellation approach describedabove, the two light sources emitting at wavelengths λ₁ and λ₂ can beconfigured to operate alternately with short periods of no illuminationbetween, to allow blank frames to be collected. In a simple form of thisembodiment, the illumination pattern and receiver can be synchronised tooperate as follows:

Illumination λ₁ off λ₂ off λ₁ Off λ₂ Receiver λ₁ blank λ₂ blank λ₁ blankλ₂ frame frame frame frame

Alternatively a more complex system could be implemented that usedseparate image capture chips for each wavelength, or which usescontinuous illumination and selectively filters the received light togenerate on and off frames at each wavelength.

Microcontroller 424 analyses the data, and determines which portion ofthe data contains information most strongly related to reflectedultraviolet light 414 and reflected infrared light 416 respectively asdescribed above.

Particle detection algorithms could then be applied independently on thereceived UV light and received IR light as described above. However, itis preferred that the two wavelengths are chosen such that they displaydifferent behaviour in the presence of particles to be detected, e.g.smoke particles. In this way the relative change in the received lightat the two (or more) wavelengths can be used to give an indication ofwhat has caused attenuation of the beam. If the relative obscuration ofthe received beams drops below a predetermined threshold then an alarmcan be raised.

Attenuation of a light beam in air is produced primarily by the effectof some of the light being scattered off-axis due to interaction withairborne particles. UV light is scattered relatively strongly by smallparticles, e.g. smoke, and IR is scattered less by such particles; thusin a smoke detector the IR beam can be used as a reference beam for theprimary UV smoke detection beam. In this example, both the UV and IRbeams will be equally sensitive to variations in received intensity thatare caused by things like drift in the system, soiling of the optics ofthe system, a large foreign object passing through the beam (e.g. a birdetc.) or relatively large nuisance particles such as dust, but the UVlight will be more severely attenuated by smoke which is typicallydominated by small particles. By carefully selecting the wavelengthsused in the system the desired particle size selectivity can be chosen.The present examples use an IR wavelength of 850 nm as a referencewavelength, however a longer wavelength such as 1500 nm may be used insome embodiments. Similarly the shorter wavelength beam can be madeshorter, say 200 nm to achieve greater sensitivity to smaller particles.Other wavelengths that are either longer or shorter can also be used.However the cost of implementing the emitter and receiver in suchsystems can make them prohibitive in most applications.

FIGS. 5A and 5B illustrate schematically the received light intensity ofa system at two wavelengths over time. In these figures the receivedlight level of UV light is illustrated by curve 1402 and the receivedlight level of IR light is illustrated by curve 1404. Generally, thereceived light intensity of the two beams vary in a similar manner overtime, and hence their ratio will be substantially constant over time. Attime 506, the two curves 1402 and 1404 diverge. This indicates that anevent has happened that has caused a greater attenuation of the UV beamthan the IR beam. Consequently the ratio of received UR and IR radiationwill move away from its substantially constant state.

Because of the properties of UV and IR radiation mentioned above, thisindicates that small particles, like smoke have entered the beam pathand caused attenuation and the microprocessor can be configured toindicate that smoke has been detected.

FIG. 5B illustrates the case where the beam is attenuated by a differentcause. At time 510, the received intensity of both beams is greatlyreduced. This indicates that the cause of the obscuration is notwavelength dependent and is likely to be either an alignment problem orlarge foreign body entering the beam.

As can be seen in this simple example, the use of a reference beam and aprimary detection beam can allow a distinction to be drawn between alikely particle detection event an another cause of beam attenuation.

The wavelengths of light mentioned here are given as examples only, andthe inventors anticipate that other wavelengths could be chosen thatcould adapt the system to detect certain types of particles. In aparticularly preferred embodiment the reference beam would not beabsorbed or scattered at all by the particles or interest, but would beattenuated by all other events. In this case the reference beam wouldgive an indication of structural or foreign body effects on the systemonly.

However, the present inventors have determined that in certaincircumstances performing smoke detection at these two wavelengths andthen subtracting a received signal at one wavelength from the receivedsignal at the other wavelength, or taking the ratio of received signalsat the two wavelengths, may be prone to failure in the presence ofcertain types of particles or clouds of particles having a certainparticle size distribution.

For example, a smoke detection test has been performed on a twowavelength smoke detection system, in which smoke was generated usingthe following set up. A white cotton towel was closely wrapped around anelectrical element and the element and towel placed in a receptacle.When electricity was passed through the element a large amount of smokewas produced. The smoke from this source was introduced to an apparatusmeasuring light transmission at violet (405 nm) and infrared (850 nm)wavelengths however, it was found that these wavelengths were affectedsubstantially equally, making a smoke detector dependent upon adifferential or ratiometric measurement ineffective. In contrast a smokedetector operating at a single infrared wavelength easily detected thissmoke.

FIG. 11A illustrates an example of a smoke detector response at twowavelengths λ₁ and λ₂. As can be seen the received light intensity attwo wavelengths varies over time but initially is substantially flat andequal at the two wavelengths. At the time t₁ smoke (generated in themanner described above) enters the detector and the received lightintensity at each of the wavelengths decreased substantially. However,unlike the case depicted in FIG. 5 a the response at both wavelengths λ₁and λ₂ decrease in concert after time t₁.

This trend can be seen in FIG. 11B which indicates a comparison of theoutput of the smoke detector at two wavelengths λ₁ and λ₂ (e.g. λ₁=405nm and λ₂=850 nm) over the same timescale as FIG. 11A. The twowavelength comparison could be any known comparative measure, such asthe received light intensity level at λ₂ subtracted from the receivedlight intensity level at λ₁, or the ratio of these values, or some othermeasure. As can be seen, since the responses at λ₁ and λ₂ remainsubstantially the same the comparison curve in FIG. 11B does not varyfar from the central position indicating that the response at wavelengthλ₁ is the same as the response at wavelength 2. In the case where thesmoke detector is configured to enter an alarm condition when thecomparison value reaches a pre-determined threshold, say T₁, thesituation illustrated in FIG. 11A would not cause an alarm to be raised.In ordinary operation the inventors have determined that a thresholdreflecting a change in relative obscuration of between 10% and 50% workswell. However the desired threshold level can be set to achieve abalance between false alarms and sensitivity.

The present inventors have devised two methods of addressing theshortcomings, which may be used either alone or together, with theembodiments of a particle detector as described above, or with othertypes of particle detector, including detectors which detect thepresence of particles on the basis of the received scattered light ineither a forward or backward scattering geometry) to avoid theabovementioned drawbacks.

In a one exemplary embodiment the present inventors have determined thatsmoke produced in the manner described above can be better detectedusing a reference wavelength and that the system can be augmented with athird wavelength light emitter to emit a third beam of light. Theinventors have determined experimentally that a beam in a wavelengthband centred at about 540 nanometres is unexpectedly unaffected byparticles in the smoke test described above.

FIG. 12A illustrates an illustrative plot of the response of a smokedetector operating at 3 wavelengths indicated as λ₁, λ₂, and λ₃ (e.g.λ₁=405 nm, λ₂=850 nm and λ₃=520 nm) in the presence of smoke generatedin the manner described above where λ₃ is in the green portion of thevisible light spectrum. In this case, the plots for λ₁ and λ₂ are thesame as in FIG. 11A, however as can be seen the plot for λ₃ is quitedifferent. In this regard, initially, ie. before time t₁, the plot forλ₁ is substantially the same as for λ₁ and λ₂. The corresponding portionof FIG. 12B, which shows the comparison between the plot of λ₁ and λ₃,is substantially flat and varies closely around the λ₁=λ₃ line.

After time t₁ at which point the smoke is introduced into the detector,the plots for λ₁ and λ₂ drop off together, however the plot for λ₃decreases in a much slower fashion. Accordingly, as seen in FIG. 12B thecomparison between λ₃ and λ₁ increases and eventually crosses the alarmthreshold t₁. As will be appreciated by those skilled in the art asimilar curve would be generated by comparing the λ₃ response with theλ₂ response.

Thus, it can be seen that by augmenting a two wavelength system with atleast one reference wavelength (e.g. a green wavelength) in a particledetector of the type described herein, particle detection events whichmay otherwise go undetected may be detected.

In an alternative form, a two wavelength system including either theillumination and only one other colour illumination could be used,rather than a three (or more) wavelength system as described above.

FIG. 13 illustrates a second mechanism determined by the presentinventors which may be used to ameliorate the disadvantages of the priorart in detecting such smoke. This approach is contrary to previousapproaches employed in smoke detectors using multiple wavelengths.Certain aspirated or point detectors on the market which use multiplewavelengths of light to detect smoke to either subtract the receiversignal one wavelength from that detected at the other wavelength, or totake a ratio of the output of the smoke detector at the two wavelengthsto detect the presence of smoke. However, as can be seen from the abovethis approach fails to detect smoke when both wavelengths are equally(or proportionately) attenuated. As an illustration, internationalpatent application WO2008/064396 in the name of Siemens Schweiz AGdescribes a multiple wavelength smoke detector which uses a shortwavelength signal to enhance the detection of small particles in theevent that the short wavelength response is substantially greater thanthe long wavelength response. However, in the case where the response ateach of the wavelengths is substantially similar e.g. where one responseis between 60% and 95% of the response at the other wavelength, theinventors teach that ratio of the responses at the two wavelengths isused. In the presence of products of combustion demonstrating anunusually high proportion of large particles compared to smallparticles, such as may be generated by heating materials in a restrictedair supply, because the response at both of the wavelengths issubstantially the same, the detector would always use the comparisonmeasurement and thus never go into alarm and the detector would fail todetect the smoke.

The present inventors have determined that this problem can be overcomeby applying a fallback detection threshold which is used to trigger analarm, irrespective of the value of the comparison between the detectorresponse at the two wavelengths.

Thus in FIG. 13, the threshold T₂ is set, and once the received lightintensity at either or both of λ₁ and λ₂ drops below that threshold analarm condition is indicated.

Such a threshold may potentially cause false alarms if the smokedetection beam is obscured by a foreign body, however this risk can beminimised by other means, such as by analysing the rate of change of theobscuration signals or applying suitable alarm delays etc. As will beappreciated, a solid body will typically cause a sharp obscurationchange, whereas a smoke plume will typically build up somewhat moreslowly and have a slower rate of change at each wavelength. Moreover, byaveraging the obscuration over a short period of time, transientobscuration, for example as may be caused by a bird flying through thebeam, can be largely ignored.

FIG. 6 depicts an embodiment capable of monitoring multiple targetssimultaneously. According to this embodiment, the detector 600 includesa light emitter 602, a receiver 604, a first target 606, and a secondtarget 608 acting in co-operation to detect smoke in monitored area 610.Target 606 reflects incident light 612, resulting in reflected light 614returning to receiver 604. Target 607 reflects incident light 616,resulting in reflected light 618 returning to receiver 604.

As with the previous embodiment, the receiver 604 includesmicrocontroller 624 for processing image data. Microcontroller 624analyses the data, and determines which portion of the data containsinformation most strongly related to reflected light 614 and reflectedlight 618 respectively. At the conclusion of this decision process, themicrocontroller 624 will have selected two portions of data,corresponding to respective individual pixels or respective groups ofpixels read from its image sensor, that can most reliably be used tomeasure the intensity of reflected light 614 and reflected light 618respectively.

In this way the system 600 can, by the addition of only an additionaltarget, perform the function of two beam detectors. A person skilled inthe art would appreciate that this principle could be extended toinclude any number of targets and reflected light beams.

FIG. 7 illustrates yet another embodiment of the present invention. Inthis example the system 700 includes only a receiver 704 and a lightemitter 706 placed on opposing sides of the monitored area 708. In thiscase the light emitter 706 is the light source imaged by the receiver.Most preferably the light emitter is a battery powered unit, includingone or more LEDs or other light emitting elements which are adapted toemit one or more beam of light across the monitored area 708, althoughother light sources (e.g. powered by mains power, or connected to thereceiver by a data cable) could be used. The light emitter 706 ispositioned within the field of view of the receiver 704 and is adaptedto emit a broad beam (or cone) of light in a volume that includes thereceiver 704. The receiver 704 is adapted to process received light (atone or more wavelengths) in the same manner as described above. In thiscase the microcontroller is adapted to identify those pixels of theimage sensor on which light emitted from the light source directlyimpinges. Particle detection based on the measured obscuration of thereceived beam then proceeds as described in connection with the previousembodiments. As will be appreciated, the light source can emit light atmultiple wavelengths (e.g. by including multiple LEDs or multicolourLEDs or a broad band light source).

In the preferred embodiment the remote light sources are independent ofeach other and free-running i.e. operate independently of the lightreceiver (that is, there are no wires or optical communications channelsfor communication between the receiver and light source(s)). In thisembodiment the receiver needs to identify the timing of each lightsource. It can then go into a process of altering and synchronising itsown frame rate with the light sources. This synchronisation will need tobe performed for each light source independently and the frame ratecontinuously adjusted to allow phase synchronisation with each lightsource in turn.

In a more complicated embodiment the camera could communicate with theremote light source(s) to synchronise the camera frame rate with theillumination modulation of the light sources.

A preferred synchronisation scheme operates as follows. Initially thebeacons are turned on and generate light beams according to theirmodulation scheme at an unknown rate. The receiver is configured tooperate continuously and identify the pixel or group pixels of on theimage sensor corresponding to each light source. Once this is performed,the receiver can identify the modulation rate of each light source, andadjust either or both of the phase and frame rate of the shutter of thereceiver accordingly.

In embodiments of the invention described below, which use a scanningcamera or light source, the frame rate and phase of the receiver, andalso the modulation rate of the light sources, can be determined tomatch the scanning rate of the system.

In a preferred embodiment of the present invention, the system will bepowered, from the fire alarm loop, thus minimising the installationcost. This minimises the installation costs of the device in that itobviates the need for dedicated wiring for supplying power or datacommunication between the emitters and receiver. However, the fire alarmloop usually only provides a very small amount of DC electrical powerfor the detector. For example, an average power consumption of about 50mW may be desirable for such a detector. However with current technologythe power consumed during video capture and processing will be far abovethe 50 mW that is available from the loop. To address this problem aseparate power supply could be used, but this is costly since standardsfor fire safety equipment are onerous, e.g. they require a fullyapproved and supervised battery backed supply, and fixed mains wiring.

In order to reduce power consumption at the receiver end it is possibleto remotely mount the light sources from the receiver and power thelight sources using a battery. This is made possible by using a lowpower light source such as a LED. Most preferably the light source ismodulated with a relatively low duty cycle to extend battery life.

A noted above, when a remotely mounted light source is used there is noneed for a reflective target as the remote light source directlyilluminates the receiver. However, it can be advantageous to use ahybrid system in which a primary light source is mounted remotely fromthe receiver, and transmits a beam of light back towards the receiveracross the region being monitored, and a second light source is mountedon the receiver. Using such an arrangement, an initial, primary smokedetection can be performed using the remotely mounted light sourcehowever, when a predetermined smoke detection threshold (e.g. anobscuration threshold) is reached the receiver mounted light source(s)can be activated. In such a scheme a reflective target will be needed toreflect the beam of the receiver mounted light source back to thereceiver for detection. In such a system the receiver-mounted lightsource can operate at multiple wavelengths to implement multiplewavelength detection as described above. The receiver mounted lightsources may operate at the same or different wavelengths to the lightsources mounted on the beacon.

FIG. 14 illustrates an exemplary combined light source and targetarrangement. The beacon 1800 includes a retroreflective target portion1802 and a light source 1804. FIG. 15 illustrates schematically a cutaway side view of the beacon 1800 to better illustrate its construction.The lower half of the beacon 1800 includes a retroreflector, in the formof a corner cube 1806. As will be known to those skilled in the art acorner cube typically includes one or more reflective arrangementshaving adjacent faces meeting at an internal angle of 90°. Using such anarrangement, light is reflected away from the reflector in a directionparallel to the incoming beam. The top part of the beacon 1800 includesa light source 1804. The light source 1804 is illuminated using an LED1808 which is connected to drive circuitry 1810 that is powered bybattery 1812. The light emitted by the LED 1808 can be passed through anoptical system, indicated as lens 1814. As can be seen a device of thistype need not be connected to any external power source or linked via acommunications line back to the receiver.

In some instances the lens or window of a transmitter or receiver maybecome obscured due to deposition of water molecules as a condensate onthe surface of lens or window. There are a range of possible approachesto avoid obscuration of the lens in this way. Using FIG. 15 as anexample, in one embodiment a heating device is provided within, orproximate to the lens 1814. The heating device operates to increase thetemperature of the lens and air within the housing 1814 and assists inreducing obscuration due to condensation. In alternative embodiments, adesiccant or other hygroscopic substance is provided within the beacon1800 to absorb moisture from the air and thus reduce the likelihood ofcondensation. As a person skilled in the art would appreciate, eitherapproach is applicable with certain modifications to the receiver 704.

FIG. 16 illustrates a further embodiment of the beacon 100 in accordancewith an embodiment of the invention.

In this embodiment the beacon 100 includes a retroreflective portion 102and a light source portion 104. However, this embodiment differs fromthat of FIGS. 14 and 15 in that two LED's 106 and 108 are provided. Eachof the LED's 106 and 108 can generate a light beam at a differentwavelength to enable the particle detector to be operated at multiplewavelengths in the manner described above.

Such a beacon can be used in an embodiment of the present inventionemploying more than two wavelengths of light for particle detection.

Because the beacons 1800 and 100 are not connected to an external powersource or the receiver via the communications line the illumination ofthe LED's will typically be modulated such that the LEDs blink from timeto time to intermittently emit a beam of light across the region beingmonitored. FIGS. 17 and 28 show modulation schemes suitable for use inan embodiment of the present invention. Using such a modulation schemebattery life of the remotely mounted beacon can be extended and regularmonitoring of particle density in the space being monitored can beperformed.

Because the beacons 1800 and 1000 are battery powered, it is necessaryto monitor the power remaining in the battery of the beacon. In order toautomatically perform this task the beacon can be programmed to changeits illumination modulation when a lower battery state is reached. Forexample, instead of using a modulation scheme 1100, an alternativemodulation scheme e.g. scheme 1102 can be employed once the batteryvoltage drops below a predetermined level. The receiver can beprogrammed to identify a change in the modulation pattern of the beaconrequires new batteries to be installed.

The modulation scheme of the beacon can be switched temporarily orintermittently to the “low battery” modulation scheme 1102 to allow thesystem to continue operating with full detection capacity.Alternatively, the low battery modulation scheme can be maintained.Whilst this scheme reduces the duty cycle of the LED to prolong batterylife further it also halves the number of particle detection readingsthat are able to be made in the given time period. However, even at thisreduced duty cycle it may still be possible to adequately detectparticles in the region being monitored.

In some implementations of the present invention in which the region tobe monitored greatly exceeds the field of view of the receiver it ispossible to implement a scanning receiver system. FIG. 18 illustratessuch a system. In this example a region to be monitored 1202 has aplurality of beacons 1204 to 1214 arranged around its perimeter. In onecorner of the room a receiver 1216 is mounted. The receiver 1216 has afield of view defined by sector 1218. The sector 1218 is relativelynarrow and does not encompass the entire region to be monitored and isinsufficient to simultaneously view all of the beacons 1204 to 1214. Inorder to overcome this short coming, the mounting means of the receiver1216 is configured to scan the receiver's field of view from one side ofthe room to the other through 90°. For example, the receiver can bepanned from a position 1220 in which it can view beacon 1204 to aposition 1222 in which it can view beacon 1214. Such a system can beadapted to cover different geometries, e.g. by centrally mounting thecamera and rotating it through 360° to view transmitters mounted on allwalls of the building. In a further alternative, a centrally mountedstatic receiver with 360° field of view could be used instead of arotating element.

The detection software of the receiver is synchronised with the scanningto determine which of the beacons 1204 to 1214 falls within its field ofview at any given time, when using a beacon of the type illustrated inFIG. 14 or 16 or a remotely mounted target. Alternatively, a receivermounted light source with a relatively narrow field of view can also bescanned across the region being monitored in synchronisation with thereceiver.

FIG. 19 illustrates an exemplary mechanism for scanning both field ofview of a receiver and the beam of a light source. The mountingmechanism 1300 includes a receiver 1302 and light source 1304. A pair ofrotating mirrors 1306 and 1308 are mounted between them and driven by adrive mechanism 1310.

The rotating mirrors in this example are shaped as a square pyramid androtate synchronously with each other. The receiver 1304 views a face ofthe rotating mirror and as the mirror rotates the field of view 1312 ofthe receiver 1302 sweeps through 90° repeatedly. The light source 1304is similarly mounted with respect to the mirror 1308 and as it rotates,the field of illumination 1314 of the light source 1304 also sweepsthrough 90°. Because the mirrors 1306 and 1308 are accurately alignedwith each other the field of illumination 1314 and the field of view1312 coincide at the point of the reflective target and are swepttogether. As will be appreciated by those skilled in the art, the anglesswept out by the mechanism of FIG. 19 can be adjusted by changing thenumber of faces on the mirrors 1306 and 1308. Moreover, in a preferredembodiment the rate of rotation of the mirrors 1306 and 1308 can becontrolled to enable synchronisation with the frame rate of the receiver1302 if desired.

FIG. 20 illustrates a further embodiment of the present invention inwhich a centrally mounted camera and light emitter arrangement are usedto detect particles in a region 1408. The camera and light emitterarrangement 1410 is preferably mounted to the ceiling of the room beingmonitored 1408 and is adapted to sweep a full 360° around the room. Thefield of illumination of the receiver-mounted light sources coincideswith the field of view of the receiver. As the receiver and lightemitter arrangement 1410 sweep around the room the plurality ofreflective targets 1406 are sequentially illuminated. In effect theparticle detection system 1400 operates as a series of radial beamdetectors centred at the middle of the room which are sequentiallyoperated to detect smoke around the room. Of course light emitters couldbe used in place of reflective targets 1406, in which case thearrangement 1410 need not carry a light emitter.

In embodiments of the present invention which use a remotely mountedbeacon it can be advantageous to have the light source mounted on thebeacon emit a relatively narrow beam of radiation. The use of a narrowbeam of radiation increases the intensity of the radiation within thebeam for a given level of power use which increases the signal receivedat the receiver. However, the use of a narrow beam light emitterincreases the need for alignment of the light source and the receiver.It should be noted however that a preferred beam divergence of between5° and 10° is permissible and accordingly that alignment below thistolerance is not needed.

In order to facilitate alignment of the light source with the receiver,the inventors have proposed several alignment mechanisms. FIG. 21illustrates a beacon 1500 including a first alignment mechanismaccording to an embodiment of the present invention. The beacon 1500includes a beacon housing 1502 and is mounted on a bracket 1504. Thebeacon housing 1502 is rotatable with respect to the bracket 1504 toallow alignment of the receiver with respect to the bracket. In thisembodiment, the beacon 1500 is provided with an indicator dial 1506 toassist with alignment during installation. A close-up of the top of thebeacon 1500 in shown in FIG. 22 and better illustrates the operation ofthe indicator dial 1506. The indicator dial 1506 includes a centralportion 1508 which is in a fixed angular relationship with respect tothe housing of the beacon 1502 and includes an indicator arrow 1510which is aligned with the centre line of the field of illumination 1512of the light source housed within the beacon. The indicator dial 1506additionally includes a series of angular graduated markings 1514 whichindicate an angular position with respect to the mounting plane of thebracket 1504.

Typically the geometry of a smoke detection system installationaccording to an embodiment of the present invention will be known beforefinal installation takes place. Accordingly, the orientation andposition of a beacon with respect to the receiver should be known. Inthis case, the installer can simply calculate the appropriate angle atwhich to set the beacon with respect to its mounting bracket and simplyalign the beacon with respect to the bracket such that the arrow 510 onthe dial aligns with the appropriate marking 1514 on the dial face.

FIGS. 23 and 24 illustrate a further alignment mechanism usable in anembodiment of the present invention. In this embodiment the beacon 1700is mounted on a mounting bracket 1702 in a manner that allows it toswivel about its point of attachment. The alignment of the beacon 1700can be determined by attaching a removable sight mechanism 1704 to thebeacon 1700. The sight mechanism 1704 operates similar to a gun sightand includes a viewing means such as eye piece 1706 and a site marker1708. In use, after mounting the bracket 1702 to its support surface theinstaller can change the angle orientation of the beacon 1700 bypivoting it in the bracket such that the receiver is aligned with thesight attached to the beacon. After installation the sight can bedetached from the beacon and used to align other beacons which form partof the smoke detection system.

FIGS. 25 and 25A illustrate an alternative beacon arrangement which canbe used in an embodiment of the present invention. For clarity a lightsource portion of a beacon 1900 is illustrated. However, the beacon mayinclude a retroreflective portion as indicated in previous embodiments.

In this beacon 1900 the light source is formed by a plurality of lightemitters, for example LEDs 1902,1904. Each of the light emittersproduces a beam of light, such as beam of light 1906 produced by thelight source 1902, which has a relatively narrow dispersion pattern.Preferably, the illumination produced by neighbouring light sourcesoverlap to enable illumination over a wide field of illumination asindicated at 1908. In use, once the beacon 1900 is mounted to a surfacethe individual light emitter which is best aligned with the receiver canbe used to form a light beam directed toward that receiver. In a systemin which multiple receivers are used to monitor the beacon 1900 two ormore of the individual light emitters 1902,1904 can be illuminated todefine the separate beams directed to the individual receivers.

Upon set up of the system the operator can manually select theindividual light emitter which is most closely aligned with the receiveror an automatic light source selection algorithm can be employed. Forexample, initially all light sources can be turned on such that thebeacon can be identified within the field of view of the receiver andthen the light sources can be sequentially turned off (or on again) in apattern to identify which of the individual light sources 1902 or 1904best illuminates the receiver.

The light source may be configured to illuminate beams over variousspatial patterns. For example, FIGS. 25 and 25A show emitters having asemi-circular profile, with each emitter being positioned on a point ona circumference of a semicircle. However, other configurations arepossible. For example, further light sources may be added such that thelight source extends both vertically and horizontally to differentextents. In the embodiment of FIG. 25A, light emitters are arrangedhemispherically in this way, a beacon can be chosen with an additionaldegree of freedom when compared to a linear/planar arrangement. Otherembodiments are possible including arrangements of light emitters inother geometric shapes, surfaces or volumes.

FIG. 26 illustrates another type of beacon which can be employed in theembodiment of the present invention. In this case the beacon 2000includes two individual light sources 2002 and 2004 which transmit lightat different wavelengths. During a set up phase the first light source2002 can be turned on and off with a modulation scheme indicated at 2006and the light source 2004 can be illuminated with a modulation scheme2008.

Because it is necessary for the receiver to receive light from bothlight sources at the same time it is necessary for the receiver to be inthe field of illumination of both light sources 2002 and 2004, that isthe receiver must be aligned within region 2010. During set up, it ispossible to use the receiver to determine whether the beacon iscorrectly aligned with the receiver in the following manner. Firstly,the light sources 2002 and 2004 are illuminated with the modulationpatterns indicated at 2006 and 2008. If the beacon 2000 is correctlyaligned with the receiver the receiver will lie in the region 2010.Because the modulation schemes 2006 and 2008 are shaped in acomplementary fashion i.e. when one is on the other is off, and they aredistinguishable from each other by their modulation patterns, thereceiver should receive a constant “on signal” when it is correctlyaligned. On the other hand, if the beacon is aligned such that thereceiver lies in region 2012 the pattern of received light will resemblethe modulation scheme 2006. If the received light appears to bemodulated with the pattern as indicated by modulation pattern 2008 thereceiver lies within the region 2014.

Thus the system is able to tell the installer whether the beacon 2000 iscorrectly aligned with the receiver, and if it is not it can tell theoperator in which direction the beacon should be adjusted to correctlyalign the beacon with the receiver.

As will be appreciated with the embodiments of FIGS. 25 and 26 a lineararrangement of light emitters is illustrated. However the array of lightsources 1902,1904,2002,2004 can be expanded in two dimensions to allowcorrect alignment to be achieved in the vertical and horizontal planesas illustrated in FIG. 25A. FIG. 25A shows a beacon 1920 including LEDs1922, 1924 projecting beams which diverge from each other in twodimensions.

Returning now to FIG. 8, which shows a particle detector according to anembodiment of the present invention which has been extended to includesix beam detectors. The system 800 is configured to monitor a space 801using a single receiver 802 to monitor six targets 804, 806, 808, 810,812 and 814. Light is emitted from a receiver mounted light source (notshown). The light source emits light over a 90° sector illuminating theentire space between the lines 816. The receiver 802 also has asimilarly wide field of view, which covers approximately 90°.

The reflected light from each of the targets 804 to 814 defines sixbeams 818, 820, 822, 824, 826 and 828. Each of beams 818 to 828 isdirected back to the receiver 802 by a respective one of the targets 804to 814. As described above, each of these beams will form an image on adifferent pixel or group of pixels on the image sensor of the receiver802 and can thereby define independent beam detectors. By providing anarray of beam detectors radiating out from a corner of the space 801 theentire room can be monitored. Moreover, since each of the beam detectorsoperates effectively independently from each other a measure ofaddressability can be achieved. For example, consider a small, localisedsmoke plume 830 which forms in a part of the room. Initially, this smokeplume 830 will not necessarily intersect with a beam of the beamdetector however, as it spreads to form smoke plume 832 it willintersect with beam 820 and the beam detector formed by the lightsource, reflector 806 and light sensor 802 will detect this smoke plume.Thus it can be determined that smoke is being detected somewhere alongthe line of beam 820. In the event that the plume spreads further, sayto form smoke plume 834, the plume 834 will additionally intersect thebeam 818 and the smoke detector formed by the light source, reflector804 and light receiver 802 will also detect smoke. This can indicatethat firstly the smoke plume has increased in size, and secondly thatthe smoke plume (or smoke plumes) have occurred somewhere along thelines of beams 820 and 818.

As will be appreciated by those skilled in the art, each of the beamdetectors can have independent alarm logic and be independentlyidentified on the fire alarm loop and be configured to separatelytrigger an alarm.

FIG. 9 illustrates a system 900 which provides enhanced addressability.The system 900 includes each of the components of the system of FIG. 8and also includes an additional receiver (and associated light sources)902. The system 900 also includes three additional reflective targets904, 906 and 908. The field of view of the receiver 902 is defined bylines 909 and substantially covers the entire space 901. Thus, thereceiver 902 can see six reflectors 904, 906, 908, 804, 806 and 808 inits field of view. Accordingly, the receiver 902, its light sources andits viewable reflectors form six beam detectors defined by beams 910,912, 914, 916, 918 and 920. As can be seen, these beam detectorsintersect with the beams received by the light receiver 902.

By providing intersecting beam detectors the addressability throughoutthe monitored area 901 is greatly enhanced. Take once again, a smallsmoke plume 830. When it initially forms, it intersects the beam 916formed by the receiver 902, its light sources and reflector 804. As itincreases in size over time to form smoke plume 832 the smoke plume 832also intersects beam 820 formed by the receiver 802, its light sources,and the reflector 806. Thus, the position of the smoke plume 832 can belocalised to the intersection between beams 916 and 820. As the smokeplume increases in size, its growth can be more accurately determined asit additionally intersects with beam 818 and will be detected by thebeam detector defined by the receiver 802, its light sources and thereflector 804. However, it should be noted that as it does not intersectany other beam it can be determined that the smoke plume 834 is growingin a particular defined region.

In this embodiment in addition to each beam being independentlyaddressable each intersection point can be nominated as an addressingpoint on a fire alarm loop or similar system and the correlation betweendetections on each of the independent beam detectors can be determinedin software to output a localised position of smoke detection. In thisway, the intersecting beams each act as a virtual point detectordetecting smoke at the point of intersection.

It will be appreciated that the embodiment of FIG. 9 enables a greatincrease in addressability with the addition only of a single receiverand through additional targets over the system of FIG. 8. In this case,27 unique points can be addressed by the system.

Whilst the description herein has discussed intersecting beams, thebeams need not actually intersect, but merely pass nearby each other sothat they monitor a substantially common location within the monitoredregion.

FIG. 10 shows another system 1000 which is able to provideaddressability. In this embodiment, the system of FIG. 8 has beenaugmented with a plurality of additional reflective targets 1002, 1004,1006, 1008, 1010, 1012 and 1014. The reflective target e.g. 1002 can bethe type illustrated in FIG. 10A.

In FIG. 10A the reflective target 1050 includes a retroreflective targetsurface 1052 mounted on a mounting bracket 1054. The mounting bracket1054 is preferably adapted to be mounted to ceiling of the space beingmonitored 1001 such that the reflective surface 1052 of the target 1050hangs downward and is illuminated by a light source of the detector andis also in the field of view of the receiver.

By placing reflective targets 1002 to 1014 at intermediate positionsacross the region being monitored 1001, addressability along the lengthof the beams can be achieved. In this embodiment, the reflectors 1002 to1012 have been placed close by a corresponding full length beam 818 to828. Thus a smoke plume which intersects beam 818 is likely to alsointersect the beam 1016 which is reflected by reflector 1002 if thatsmoke plume is positioned between the reflector 1002 and the receiver802. If the smoke plume occurs further away from the receiver 802 thanthe reflector 1002 then only the beam detector on beam 818 will detectsmoke. Furthermore, pendant reflectors can be placed at other positions,for example midway between other beams e.g. pendant 1014 which reflectorbeam 1018 midway between beams 818 and 820. As discussed in the previousembodiments a small smoke plume 830 which initially forms and does notintersect any of the beams will not be detected by such a system.However, once it has grown to plume 832 it will intersect the outer partof beam 820 and be detected by the particle detector defined by thereceiver 802, its associated light source, reflector 806. However,because it is further away from the receiver than the reflector 1004 itwill not intersect beam 1020 and thus will not be detected by the beamdetector defined by that reflector. Accordingly, the smoke plume can bedetermined to be at some portion on the outermost part of beam 820. Asthe plume further increases in size to form plume 834 it will intersectthree beams, namely beam 820, beam 1018 and the outer part of beam 818.Accordingly, it can be determined with high certainty that the smokeplume 834 is forming on the outer parts of beams 818 and 820 and alsointersects beam 1018. It can be seen that by placing a plurality of suchintermediate reflectors within the field of the receiver 802addressability of the system can be greatly enhanced. Such an embodimentcan be implemented to great effect in an environment which has multipleroof beams across the space being monitored as each roof beam willeffectively define a plane on which reflectors may be convenientlymounted and provide depth addressability along the beam. In thisembodiment, the light receiver 802 will need to be placed out of theplane defined by the plurality of beams in order to be able to view eachof the beams separately. Clearly any of the addressing schemes describedherein could be implemented with remotely mounted light emitters ratherthan reflective targets, as illustrated. Moreover, a combination of theaddressing schemes of FIGS. 9 and 10 could also be used.

The present inventors have realised that since smoke detectors do notneed to respond instantaneously, acceptable average power consumptioncould be obtained by activating the video capture and/or videoprocessing subsystems of the smoke detector intermittently, interspersedwith periods when processing and capture is suspended. Thus the systemcan enter a “freeze” state in which it is designed to consume verylittle or no power. A first way of achieving this solution is to providethe video processing subsystem of the particle detector with a simpletimer unit which operates to activate the video capture and processingsubsystems intermittently. However, in the preferred form of the systemthe transmitter 324 is not powered from the loop or other mains power,but is battery powered and is preferably not connected to the receiver322 or in high speed communication with it. Consequently the transmitter324 must emit light at only very low duty cycle to conserve power. Insuch a system the timing of each transmitted burst of light may neither,be controlled by the receiver or synchronised with any other receiverwhich may also be communicating with the same transmitter 322.

Furthermore, during the video processor “freeze” period the receiver 322may still be required to manage other functions such as servicing pollsfrom the fire alarm loop, or blinking display LEDs or the like.Therefore, using a simple timer mechanism to activate the videoprocessor and awake it from its “freeze” state is not the preferredsolution to this problem.

In a preferred form of the present invention the receiver 322 employs asecondary processor, having much lower power consumption than the videoprocessing processor, or primary processor, which is used to activatethe main processor and to deal with other functions that must continuewithout interruption when the main processor is in its “freeze” state.

FIG. 27 illustrates a schematic block diagram of a receiver 401embodying the present invention.

The receiver 401 includes an imaging chip 403, e.g., a CMOS sensormanufactured by Aptina Inc, part number MT9V034, for receiving opticalsignals from a transmitter 324. It may optionally include an opticalsystem 405 e.g. a focusing lens, such as a standard 4.5 mm, f1.4 c-mountlens, for focusing the received electro magnetic radiation onto theimaging chip in the desired manner.

The imaging chip 403 is in data communication with a controller 407which preferably is an Actel M1AGL600-V2 field programmable gate array(FPGA), and an associated memory 409 including a PC28F256P33 flash ROMfor program storage, two IS61LV51216 high-speed RAMs for image storageand two CY621777DV30L RAMs for program execution and data storage. Thecontroller's function is to control the image chip 403 and perform therequired sequence of data manipulations to carry out the functionsrequired by the detection system. The control means has sundryadditional components as required for correct operation as wellunderstood by those skilled in digital electronics design.

A second processor 413 is also provided. This processor 413 can be aTexas Instruments MSP430F2122 microcontroller or similar, and performsfunctions such as checking the health of the control means and if neededsignalling fault to external monitoring equipment if the control meansfails or if the control means, for any other reason, cannot perform itsrequired tasks. It is also responsible for the timely control of powerto the control and imaging means in order to minimize power consumption.This is performed by processor 413 de-activating the main processor 407when it is not needed and waking it up intermittently when it isrequired.

Processor 413 is also in data communication with interface means 415such as a display or user interface and is also connected to the firealarm loop to enable data communication with other equipment connectedto the fire alarm loop e.g. a fire panel.

In the preferred embodiment the interface means is used to notifyexternal monitoring equipment if an alarm or fault condition exists. Ifit is determined by the receiver that a fault exists, the interfacemeans notifies this to the monitoring equipment by opening a switchthereby interrupting the current flow out of the aforementionedmonitoring equipment. In the preferred embodiment the switch is a solidstate arrangement employing MOSFET transistors which has the benefit ofbeing activated and deactivated with very low power consumption. If itis determined by the receiver that an alarm condition exists, theinterface means notifies this to the monitoring equipment by drawingcurrent in excess of a predetermined threshold value from the monitoringequipment. In the preferred embodiment the excess current draw isachieved by the positioning of a bipolar-transistor, current-limitedshunt across the interface wires from the monitoring equipment. A totalcurrent draw of approximately 50 mA is used to signal the alarmcondition. In the preferred embodiment, power for normal operation isdrawn from the connecting wires to the monitoring equipment at aconstant current of 3 mA under non-alarm conditions.

In the preferred embodiment of the present invention the transmitter 324includes a controller to control its illumination pattern, controllingthe illumination time, sequence and intensity for each of the lightsources, e.g. infra-red and ultra-violet. For example this could be aTexas Instruments MSP430F2122 microcontroller. The microcontroller alsodetects activation of the device when first installed. In the preferredembodiment of the transmitter, the power source is a Lithium ThionylChloride battery.

In a preferred form of the present invention, during commissioning ofthe system the main processor 407 can be programmed to discover theillumination pattern of each of the light sources and over a period ofpreferably several minutes e.g. 10 minutes, determine its activationpattern. This process can be repeated for all light sources associatedwith the receiver. The low power processor 413 can use the discoveredlight source sequencing information to activate processor B at thecorrect time.

As will be appreciated, by using a system of this structure the functionof the system which must operate at all times can be controlled by thevery low power consumption processor 413 whilst the highly intensiveprocessing can be performed intermittently by the main video processor407, and in doing so the average power can be maintained at a relativelylow level.

The inventors have determined that, there are various and oftencompeting constraints associated with practical embodiments that must bedealt with when choosing the illumination pattern of the transmitter andcorresponding receiver operation to accurately acquire and track atransmitter output. For example, in some systems it is desirable to usethe rate of change of attenuation is used to distinguish faultconditions from particulate detection events. This complicates the useof long integration times discussed in the background. The preferredembodiment uses an integration period of 10 seconds for normalmeasurements, and a shorter integration period of one second is used forrate of change based fault detection.

Another constraint on system performance is the scene lighting level.For a practical system it is usually necessary to assume the scene maybe lit by sunlight for at least part of its operational life. There mayalso be limitations on the ability to use wavelength selective filterson the camera (e.g. at least cost limitations). Therefore. it will benecessary to use short exposures to avoid saturation, and still leavesufficient head room for the signal. In preferred implementations of thesystem the exposure duration is 100 us, but the optimum value willdepend on the choice of sensor, filter, lens, worst case scene lightingand the amount of headroom required for the signal.

A means of synchronising the receiver with the transmitter is alsorequired. It is preferable to achieve this without the use of additionalhardware such as a radio system or hard wiring between components.Instead in one desirable implementation the synchronisation is performedoptically using the same imaging and processing hardware that is usedfor particle detection. However, as a person skilled in the art willappreciate, the use of the same hardware for particle detection as forsynchronisation links two concerns within the system, and therebyimposes a further constraint on the possible solutions.

Another constraint within the system is due to the presence of noise.The prime noise sources in the system are camera shot noise and noisefrom light variations in the scene. Dark noise is generally not asignificant contribution for systems that must deal with full sunlight.Scene noise is dealt with very effectively by the background subtractionmethod described in our earlier patent applications. Shot noise cannotbe totally removed, as it is fundamental to the quantum detectionprocess. However, shot noise can be reduced by reducing exposure time,and also by summing fewer exposures. In the preferred embodiment,substantially all transmitter power is put into very brief flashes, witha repetition rate that still allows an adequate system response time.

For example, a flash rate of 1 per second will satisfy the response timerequirement, and a flash duration of less than 1 μs and an exposure timeof 2 μs could (in principle) be used. In practice this would be verydifficult to synchronise. In addition, the transmitter LEDs would needto handle a very high peak current to deliver the energy in such a shorttime, which in turn would increase cost. Another limitation is thedynamic range of the sensor. Putting all the power into one flash persecond could result in saturation in the sensor.

In consideration of the above factors the preferred embodiment uses anexposure of 100 μs, a flash duration of 50 μs, and a period of 300 ms.An integration length of 3 samples is used for rate of change basedfault detection. An integration length of 30 samples is used for smokemeasurements.

To perform the background cancellation techniques, the receiver alsoneeds to capture images just before and just after the flash that areused to eliminate the contribution from the scene. Ideally these “off”exposures would occur as close to the “on” exposure as possible tooptimise cancellation in the case of a time varying background. With thereceiver system used in the preferred implementation, the maximumpractical frame rate is 1000 fps, so the “off” exposures are spaced 1 mseither side of the “on” exposure.

In one form, the transmitter optical output consists of a series ofshort pulses, with a very low duty cycle. The pulses are placed to matchthe frame rate of the imaging system (e.g. 1000 fps). FIG. 28 shows anexemplary pulse sequence in relation to the sensor exposures in thereceiver. In this case the transmitter is adapted to emit light in an IRwavelength band and an IR wavelength band. This series of pulses isrepeated with a period of 300 ms.

In the example, there are 5 pulses, as follows:

-   -   Sync 1 (frame 1) 110 and Sync 2 (frame 2) 112: Sync pulses are        used to maintain synchronisation (discussed more fully later)        between the transmitter and receiver. These are pulses are        preferably made in the wavelength band which is most power        efficient. In this case the IR light source is used because it        results in lower power consumption. Moreover the longer        wavelength is more able to penetrate smoke, so synchronisation        can be maintained in a greater range of conditions. The Sync        pulses are 50 us long.    -   Ideally each synch pulses is centred in time on the leading        (sync 1) and trailing edges (sync 2) of the receiver's shutter        open period. This makes their received intensity vary with small        synchronisation errors.    -   IR (frame 5) 114 and UV (frame 7) 116: The IR and UV pulses are        used for signal level measurement (and in turn used to measure        attenuation and smoke level. They are 50 us long, which allows        for up to 25 us timing error between transmitter and receiver        without influencing the received intensity.    -   Data (frame 9) 118: The data pulse is used to transfer a small        amount of data to the receiver. The data is encoded by a either        transmitting or not transmitting the data pulse. The data pulse        has reduced amplitude to save power, and is IR for the same        reason. They are 50 us long. This system provides a 3 bps data        channel. The data may include serial number, date of        manufacture, total running time, battery status and fault        conditions. Those skilled in the art would be aware of many        alternative ways to send data in this system. These could        include pulse position encoding, pulse width encoding, and multi        level encoding schemes. Greater data rates could readily be        achieved, however the simple scheme used in the preferred        implementation is sufficient for the small amount of data        needed.

In FIG. 29, the data from the receiver during “off” frames (i.e. frameswith no corresponding transmitter output) are used for the followingpurposes:

-   -   Frame 0 & 3 are used for background cancellation of the sync        pulses    -   Frame 4 & 6 are used for background cancellation of the IR pulse    -   Frame 6 & 8 are used for background cancellation of the UV pulse    -   Frame 8 & 10 are used for background cancellation of the Data        pulse    -   (a) Spatial Search

As described above, the receiver receives each of the transmitted pulsesin the form of one or more pixels within an image frame.

However, during commissioning when the system commences operation (atleast the first time) the locations of the transmitter(s) within theimage frame must be established. This could be performed for example, bya manual process involving an operator inspecting the image, andprogramming in the co-ordinates. However, the need for special training,special tools, and long complex installation processes for installationis undesirable. In the preferred embodiment determining the location ofthe transmitters within the image frame is automated. The preformedprocess for locating transmitters operates as follows:

-   -   The system first captures a number of images at a high frame        rate and for a time sufficient to ensure that transmitter        pulses, if the transmitter is within the field of view of the        camera and pulses are transmitted during the period of capture,        will be present in one or more images.    -   The system then subtracts each pair of (temporally) adjacent        images, and takes the modulus of each pixel and then tests each        against a threshold to detect locations of large variation, at        which a transmitter may be present.    -   The system then condenses the candidate list of transmitter        locations by merging candidate points that are adjacent or        nearby. (e.g. <3 pixels apart) A centre of image method can be        used to find the centre of a set of candidate points.    -   The system then performs a trial synchronisation (using the        process described below) at each of the candidate centres to        verify that the received value at a candidate centre corresponds        to a real transmitter.    -   The system then checks that the number of transmitters matches        the expected number of transmitters. This number may be set by        pre-programming the receiver prior to installation, or by a        switch or switches mounted on or in or connected to the receiver        unit. In the preferred implementation, there is a set of        configuration DIP Switches incorporated into the receiver unit        and easily accessible only while the system is not mounted to        the wall.

The set of transmitter locations within the image is stored innon-volatile memory. The locations can be cleared by placing thereceiver into a particular mode, e.g. by setting the DIP switches to aparticular setting and powering/de-powering the receiver, or by the useof a special tool, such as a notebook PC. This is only required if atransmitter is moved from its original location or the system is to bere-installed elsewhere.

Performance limitations in the imaging system may limit the number ofpixels or lines that can be read out when operating at a high framerate. In one implementation, a maximum of 30 lines of 640 pixels can beread out in 1 ms. Therefore the first few steps of the above method needto be repeated 16 times to cover the entire 640*480 image frame.Alternatively, some embodiments employ only part of the image frame.Similarly, some embodiments use a slower frame rate. However, thepossibility of sensor saturation in bright lighting conditions generallylimits exposure time, and variations in background lighting conditionsgenerally introduce more noise if a lower frame rate is used.

The frame rate must be chosen to ensure that the transmitter pulses donot always occur in period where the shutter is closed. For example, ifthe frame rate is exactly 1000 fps, with an exposure of 100 us, and thetransmitter produces pulses on exact 1 ms boundaries, the pulses may allbe generated at times when the shutter is closed. The receiver framerate is chosen so that there is a slight difference causing a gradualphase shift, ensuring that sooner or later the pulses will fallsufficiently within a shutter open period.

In some embodiments, processing speed limitations are managed by notanalysing all of the pixels, instead only every fourth horizontal andvertical pixel are subtracted and checked, reducing processing effort bya factor of 16. Provided that the received image i.e. the image of eachtransmitter on the sensor, is spread over a sufficiently larger area(e.g. a spot having a diameter of 5 pixels), then the transmitter willstill be found reliably.

Whenever the system is powered up, either with a known set oftransmitter locations or as a part of the Spatial Search describedabove, with a set of candidate locations, a phase search and lock methodis used to establish initial synchronisation.

The major steps of this method are:

The system captures images at a high frame rate (at least a partialimage in the expected location).

The system waits for the expected pattern of pulse to appear at thecandidate entre locations.

The system uses the time of arrival of a selected pulse within theexpected pattern as a starting phase for the phase locked loop.

The system waits for stabilisation of the PLL. If no PLL lock is made,then in the case of testing candidate locations, the location is markedas spurious, otherwise when re-establishing synchronisation with a knowntransmitter location the receiver can re-try continually and assert afault until it is successful.

As with the spatial search, a small offset in the receiver frame rate isused to cause a gradual phase shift, ensuring that sooner or later thepulses will fall sufficiently within a shutter open period.

For each frame, the total intensity is calculated within a small regionof the image centred on the known or candidate location. This sequenceof intensity values is then checked for the expected pattern from thetransmitter.

The test for the expected pattern operates as follows:

After at least 9 frame intensity values have been collected, they can betested for the presence of the expected transmitter pulse sequence inthe following manner.

Given the intensity values I(n), 0<n<N,

Test for a possible transmitter signal starting with its frame 0 atframe n received

First, compute an “off frame” reference level

I ₀=(I _(R)(n+0)+I _(R)(n+3)+I _(R)(n+4)+I _(R)(n+6)+I _(R)(n+8))/5{mean of “off frames”}

Compute relative intensities

I _(R)(n+m)=I(n+m)−I ₀ for m=0 to 8

Compare with pre-determined thresholds to determine the presence orabsence of a transmitter pulse in each frame

Found = {(I_(R)(n + 1) > I_(ON))or {Sync 1 or Sync 2 pulse} (I_(R)(n +2) > I_(ON))} and (I_(R)(n + 5) > I_(ON)) and {IR pulse} (I_(R)(n + 7) >I_(ON)) and {UV pulse} (I_(R)(n + 0) < I_(OFF)) and {off frame}(I_(R)(n + 3) < I_(OFF)) and {off frame} (I_(R)(n + 4) < I_(OFF)) and{off frame} (I_(R)(n + 6) < I_(OFF)) and {off frame} (I_(R)(n + 8) <I_(OFF)) and {off frame}

Due to the random phase errors, either of the sync pulses may becompletely missing, hence the “or” in the above expression.Alternatively, the tests for the sync pulses can be omitted entirely,and the tests for the off frames can also be reduced. However, care mustbe taken to ensure that the position of the transmitter pulse sequenceis not falsely identified.

Following a positive detection, the time corresponding to the frame n isrecorded in a variable. The amplitudes of the phase pulses can be usedto trim the recorded time value to more closely represent the start ofthe sequence. This helps reduce the initial phase error that the phasedlocked loop has to deal with, and may not be required if frequencyerrors are sufficiently small.

In the preferred implementation the image capture rate 1000 fps whichmatches the transmitter timing as previously described. A shutter timeof 100 us is used.

This completes the initial synchronisation. The arrival time of the nextset of pulses can now be predicted by simply adding the knowntransmitter period to the time recorded in the previous step.

Although the transmitter period is known to the receiver (300 ms in thepreferred implementation), there will be small errors in the clockfrequencies at each end. This will inevitably cause the transmittedpulses to become misaligned with the receiver shutter open time. A PhaseLocked Loop system is used to maintain the correct phase or timing. ThePLL concept is well known so will not be described in detail. In thepreferred implementation the PLL control equations are implemented insoftware. The Phase Comparator function is based on measuring theamplitude of the phase pulses. These amplitude are calculated bysubtracting the mean of the intensities measured in the nearest offframes (frames 0 & 3). The phase error is then computed with thefollowing formula:

$ɛ = {\frac{{I_{R}(1)} - {I_{R}(2)}}{2\left( {{I_{R}(1)} + {I_{R}(2)}} \right)} \cdot T}$

where T is the width of the phase pulses.

In the case that the phase pulse amplitudes fall below a pre-determinedthreshold, the phase error is assigned a value of zero. This way noisydata is permitted into the PLL, and in practice the system is able tomaintain adequate synchronisation for at least a few minutes. Therefore,high smoke levels do not cause a synchronisation failure before an alarmcan be signalled. In the case of an obstruction, this feature allows thesystem to recover rapidly when the blockage is removed.

The PLL control equations include proportional and integral terms. Itmay not be necessary to use a differential term. In the preferredimplementation proportional gain and integrator gains of 0.3 and 0.01respectively were found to produce acceptable results. In a furthervariation, the gains can be set to larger values initially, and reducedafter the phase error is below a pre-determined threshold, thus reducingoverall lock time for a given loop bandwidth.

Phase error below +/−10 us can be used to indicate phase lock, both forthe purpose of verifying a candidate transmitter location and also forallowing normal smoke detection operation to commence.

It will be understood that the invention disclosed and defined in thisspecification extends to all alternative combinations of two or more ofthe individual aspects or features mentioned or evident from the text ordrawings. All of these different combinations constitute variousalternative aspects of the invention.

It will also be understood that the term “comprises” (or its grammaticalvariants) as used in this specification is equivalent to the term“includes” and should not be taken as excluding the presence of otherelements or features.

1. A particle detection system including; at least one light sourceadapted to illuminate a volume being monitored at least two wavelengths;a receiver having a field of view and being adapted to receive lightfrom at least one light source after said light has traversed the volumebeing monitored and being adapted to generate signals indicative of theintensity of light received at a plurality of regions within the fieldof view of the receiver; and a processor associated with the receiveradapted to process the signals generated by the receiver to correlatelight received at least two wavelengths in corresponding regions withinthe field of view of the receiver and generate an output indicative ofthe relative level of light received at the two wavelengths.
 2. Theparticle detection system as claimed in claim 1 wherein the receiverincludes a sensor having plurality of sensor elements each of whichreceives light from a respective region within the field of view of thereceiver.
 3. The particle detection system as claimed in claim 1 whereina light source can include a one or more of light emitters adapted toemit light at a respective wavelength.
 4. The particle detection systemas claimed in claim 1 wherein a light source is configured to illuminatethe volume being monitored at each of the at least two wavelengths atdifferent times. 5.-6. (canceled)
 7. The particle detection system asclaimed in claim 1 wherein the processor is adapted to determine arelative obscuration of light received at one or more pairs ofwavelengths received in the same region in the field of view of thereceiver and generate an output indicative of the presence of particlesin the volume being monitored on the basis of the relative obscuration.8. The particle detection system as claimed in claim 1 which is adaptedfor detecting particles of interest within the monitored volume, whereinthe at least two wavelengths include: at least a first wavelength whichis relatively unaffected by particles of interest, and at least a secondwavelength that is affected by at least said particles; and wherein theprocessor is adapted to process the output of the receiver at least oneof the first and second wavelengths and provide an output indicative ofwhether particles of interest are detected in said monitored region.9.-22. (canceled)
 23. The particle detection system as claimed in claim1 wherein the processor is configured to apply a second alarm conditionthat is determined on the basis of the received light level at one orboth wavelengths.
 24. The particle detection system as claimed in claim23 wherein the second alarm condition is based on a pre-condition that achange in the relative intensity of light does not cause the first alarmcondition to be met.
 25. The particle detection system as claimed inclaim 1 wherein at least one light source is battery powered.
 26. Theparticle detection system as claimed in claim 1 including at least twolight sources that are unsynchronised with each other.
 27. A method inparticle detector, including: emitting light including a first andsecond wavelength into a region being monitored; the first wavelengthbeing a wavelength whose transmission across the monitored region isrelatively unaffected by particles of interest and the second being awavelength whose transmission across the monitored region is affected byparticles of interest; receiving light at least a first and secondwavelengths after traversing the region being monitored and generating asignal indicative of the intensity of the received light at least thefirst and second wavelength; and processing the signal indicative of theintensity of the received light at least the first and second wavelengthto provide an output indicative of whether particles of interest aredetected in said monitored region.
 28. The method as claimed in claim 27wherein the step of processing the signal indicative of the intensity ofthe received light at least the first and second wavelength is based ona change in the relative intensity of the light received at the firstand second wavelengths.
 29. (canceled)
 30. The method as claimed inclaim 27 wherein in the event that the relative intensity of the lightreceived at the first and second wavelengths remains substantiallystable but an absolute intensity of the light received at one or more ofthe wavelengths meets one or more predetermined criteria the methodincludes, generating an output indicating the presence of particles ofinterest in said monitored region. 31.-53. (canceled)
 54. The particledetection system as claimed in claim 1 wherein the receiver includes acamera having a sensor comprising a plurality of pixels, and where theregions of the receiver corresponding to a respective light sourcecomprise a subset of the plurality of pixels of the sensor.
 55. Themethod as claimed in claim 27 wherein the method includes: approximatelyaligning at least one light source and receiver such that the at leastone light source illuminates the receiver; and selecting a spatialposition within the field of view of the receiver corresponding to alight source that will be used for determining received light intensitymeasurements corresponding to the light source.
 56. The method asclaimed in claim 55 wherein the method includes: tracking a region thatcorresponds to the light source over time as the geometry of the systemvaries.